Molecular hybridizations of polysomal poly(A)-contining messenger RNAs (mRNAs) of undiferentiated (S) and differentiated (P) S20Y-mouse neuroblastoma cells with their complementary DNAs (cDNAs) had shown that there were many mRNAs which were uniquely present in one of the diffeentiation steps. We have continued to develop the technology for molecular cloning of these S and P cell-specific cDNAs, using recombinant DNA techniques. We have used mouse brain poly(A)+ mRNA sequences for pilot studies and have obtained several clones of highly abundant sequences from them. We are presently ready to try cloning of S20Y sequences.